1. Field of the Invention
The present invention relates to a discharge-lamp lighting device adapted for high-frequency lighting of an electric discharge lamp by the use of a transistor inverter.
2. Description of the Related Art
Japanese laid-open patent publication No. 61-2299 discloses a transistor inverter for lighting a discharge lamp and a current transformer provided in the current path of the discharge lamp. The current transformer causes self-oscillation of the transistor inverter.
When one electrode of a discharge lamp enters the emitterless state at the end of the life of the lamp, the discharge lamp will be lighted only on one polarity of alternating-current voltage. This phenomenon is called the half-wave discharge. If the half-wave discharge occurs with the conventional discharge-lamp lighting device, the current transformer will be saturated only in one polarity because of direct-current flow therethrough. Then the transistor inverter may cause abnormal oscillation.
Japanese utility model publication No. 55-34639 discloses a discharge-lamp lighting device using a transistor inverter, in which a direct current blocking capacitor is inserted in the path of lamp current flowing through the lamp. The capacitor is required to withstand a peak value of the inverter output voltage. Thus the discharge-lamp lighting device becomes expensive and large. In addition, the capacitor intermittently discharges the stored charge through the lamp and inverter output circuit, so that a transistor or transistors in the inverter is intermittently supplied with a surge voltage. Hence the transistor or transistors of the inverter may possibly be subjected to breakdown.